A motor vehicle equipped with such anti-locking brake and drive traction regulation systems is described, for example, in the VDI Conference Report entitled "Elektronik im Kraftfahrzeug" ("Motor Vehicle Electronic Systems"), Oct. 9/10, 1986, the teachings of which are incorporated herein by reference thereto.
The primary purpose of anti-locking brake systems is to prevent undesirable locking of the braked wheels. As is well-known in the vehicle braking art, such systems are of particular use on slippery streets. Furthermore, these systems enhance the driver's capability of maintaining the steering ability of the vehicle. For this purpose, the braking torques are reduced during a time interval when a tendency to locking appears, and are subsequently increased again when such brake locking tendency disappears. To achieve this function, the activation of the brakes is controlled by a special electronic system. This electronic system is generally designed with four channels. That means that on a two-axle vehicle, for example, each individual wheel has its own electronic channel. For three-axle vehicles, six-channel electronic systems are customary, but such six-channel electronic systems are relatively complex and rather expensive. The primary reason for this is because of the relatively small quantities in which they are manufactured.
In addition to the anti-locking brake system, motor vehicle drive traction regulation systems are well-known in the vehicle. Such drive traction regulation systems prevent slipping of the drive wheels during the start-up and acceleration operating stages of the vehicle. This drive traction regulation system facilitates the start-up and improves the lateral traction of the motor vehicle during acceleration. Drive traction regulation systems are generally provided as an extension of an anti-locking braking system already installed on the motor vehicle and also incorporate the use of essential components of the anti-locking braking system, such as control valves, wheel sensors, and the electronic system.
It is customary in the operation of a drive traction regulation system to brake the wheel which slips first (low wheel), by activating the corresponding wheel brake to almost the speed of the vehicle. As a result of this "differential brake control", by means of the differential transmission, the torque of the motor is increasingly transmitted also to the second wheel running on a good road (high wheel). The motor vehicle can thus "support" itself on the high wheel and start up or accelerate as desired.
The above-mentioned braking of the low wheel is done when this wheel exceeds a specified velocity difference or traction slip, for example, about six percent. The brake on the low wheel is normally released when the values again drop below this specified threshold. A braking to less than the vehicle speed must naturally be avoided under any circumstances, as is understood by persons skilled in the braking art.
If the high wheel should also begin to slip, the output power of the motor is reduced by means of an automatic intervention in the throttle linkage of the vehicle. Such output power reduction of the motor occurs completely independently of any action on the part of the driver of the vehicle.
There is a problem, however, in addition to cost considerations, when a four-channel ABS/ASR electronic system is installed in a three-axle vehicle that is equipped to operate with one drive axle and a non-driven front or rear axle. This problem arises because there are no sensors for the wheels of one of the forward and rear axle.
In this arrangement, naturally, during a controlled start, the brake cylinders of the front or rear axle, the pressure of which is normally dependent on the brake cylinders of the drive axle, are not braked at the same time, since otherwise starting would be even more difficult.
One configuration in which a rear axle is coupled to the drive axle by means of a select-low valve is illustrated in FIG. 2 of German Patent No. DE-OS 31 01 731. Here, however, there is no ASR function performed.